Bortezomib is a modified dipeptidyl boronic acid derivative derived from leucine and phenyl alanine. The chemical name is [(1R)-3-methyl-1-[[(2S)-1-oxo-3-phenyl-2-[(pyrazinylcarbonyl)amino]propyl]amino]butyl] boronic acid and represented as follows:

U.S. Pat. No. 5,780,454 discloses Bortezomib while WO02059130 describes the mannitol ester of Bortezomib.
In U.S. Pat. No. 5,780,454, the preparation of Bortezomib is not exemplified. WO2005097809 describes large scale preparation of Bortezomib wherein the reaction is carried out in the presence of 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU), tertiary amine and in dichloromethane as a solvent and which involves solvent exchange to replace dichloromethane with ethyl acetate.
There are other patent applications such as WO2009004350, WO2009036281, WO2010146172, WO2011087822, WO2011098963 and WO2012048745 which describe various processes for the synthesis of Bortezomib.
WO2009004350 and WO2009036281 describe the condensation of N-(pyrazinylcarbonyl)-L-phenylalanine with 4,6-methano-1,3,2-benzodioxaborole-2-methanamine, hexahydro-3a,5,5-trimethyl-α-(2-methylpropyl)-(αR,3aS,4S,6S,7aR)-trifluoro acetate.
WO2010146172 describes the preparation of 4,6-methano-1,3,2-benzodioxaborole-2-methanamine, hexahydro-3a,5,5-trimethyl-α-(2-methylpropyl)-(αR,3aS,4S,6S,7aR)-trifluoro acetate.
WO2011087822 describes the preparation of Bortezomib by deprotection of (2S)-N -[(1R)-1-(1,3,6,2-dioxazaborocan-2-yl)-3-methylbutyl-3-phenyl-2-(pyrazin-2-yl formamido)propanamide.
WO2011098963 describes the preparation of Bortezomib wherein any 2 steps of last 3 steps are carried out without isolation of the intermediates.
WO2012048745 describes the preparation of Bortezomib using cyclic phosphonic acid anhydride as a coupling agent.
The prior art processes require the use of halogenated solvents, solvent exchange and tedious work up procedures and hence, result in a poor yield.
Thus, there is a need to develop an industrially feasible, economic process and which provides the product with improved yield, and improved chemical purity as well as optical purity.